Significance Statement

Energy storage devices present a noteworthy challenge in creating a robust deformable system because they must be flawlessly integrated with intelligent wearable devices such as stretchable circuits, epidermal health electronics and flexible displays. The lithium ion batteries are the preferred source of power for such deformable functional devices if they can be compressed, rolled, stretched, buckled and folded. However, selecting an active material for convectional lithium ion batteries poses a major challenge since it easily delaminates or fractures upon being deformed. Moreover, the high-density attribute of the metal restricts the energy density of the lithium ion batteries. Therefore, replacing the metallic foils with light weight, flexible and highly conductive current collector can impart flexibility to the devices and may also rise the energy density of lithium ion batteries.

In a recent paper published in Journal of Power Sources, researchers led by professor Zi Ping Wu at Jiangxi University of Science and Technology in China proposed a study on Potential threshold of anode materials for foldable lithium-ion batteries featuring carbon nanotube current collectors. They aimed at providing a principle for the selection of an active material based on flexible current collectors for foldable lithium ion batteries.

First the research team prepared flexible carbon nanotube macro-film composed of carbon nanotube bundles by chemical vapor deposition. They then fabricated the electrodes by using slurry-based preparation technique. Next, they assembled coin-type half-cells in an argon filled glove box with bare flexible carbon nanotube macro-film disks as working electrodes, lithium titanium oxide and GPE that use copper foil as current collector for loading, lithium cobalt oxide that uses aluminum foil as current collector and lithium metal as reference electrode. These cells were then assembled in a de-humidified room with moisture content less than 2%. Eventually, cyclic voltammetry measurements were carried out using standard electrochemical instrumentation.

From the set up described it was observed that the electrochemical behavior characteristic for the two electrodes, lithium cobalt oxide -aluminum and lithium cobalt oxide – Flexible carbon nanotube macro-film was similar except for the current of oxidation peak. No notable difference was observed in their CV curves and galvano-static discharge/charge profiles. It was noted that when the anode material had a potential higher than 0.9V, good performance of the flexible carbon nanotube macro-film based foldable lithium ion batteries is obtained since the lithium ion passed the potential threshold and the flexible carbon nanotube macro-film retained its electrochemical inactivity. However, if the potential anode is lower than 0.9V several free lithium ions will be constrained. In return the capacity of the flexible carbon nanotube macro-film based electrode will be lowered and the cycling performance will be poor.

The flexible carbon nanotube macro-film based foldable lithium ion batteries generally performed well experimentally. Therefore, it is expected that the mechanism of potential threshold of anode materials will provide new impetus to both industry and academia to explore the development of lithium ion batteries in flexible and foldable energy storage devices.

About the author

Dr. Wu is an associate professor at Jiangxi University of Science and Technology. He currently specializes on carbon nanotube macro-films for flexible batteries and electromagnetic shielding applications. Under his guidance, a folding insensitive and high energy density lithium-ion battery has been reported in 2015.